The invention relates generally to a method of and a press for the production of tablets and more particularly the invention relates to a method in which loose material is fed into a tablet press having a charging device including a rotary filling wheel and rotary dosing wheel, means for adjusting the rotary speed of the wheels and a die plate including a plurality of dies each cooperating with a lower die plunger the vertical working position of which is adjustable by means of a dosing cam, and further including an electronic measuring device for measuring compressing forces applied on the loose material.
In the manufacture of tablets in a press, efforts are being made to insure that the tablets have uniform weight corresponding as close as possible to a predetermined standard weight and moreover that the manufacture of individual tablets takes place under the application of uniform forces corresponding to a preset nominal value as close as possible.
The compressing force applied to the tablet is among other factors of particular importance for the reason that it determines the dissolution time of the tablets or the disintegration of the latter. If, for example, the compressing force is too high, the tablet may dissolve only after it had left the stomach and entered the intestine. The effect of such a tablet is different from the effect of a tablet of the same weight that has been produced under a lower compressing force and that consequently dissolves faster than the former tablet.
Moreover, also the firmness of the tablet depends on the compressing force. Certain firmness or solidity of the tablet is necessary for a further processing, for example in dragee kettles and in packing. For example, if the compressing force is too low the tablet disintegrates already under a small load. If, however, the compressing force is too large the tablet can split into layers.
The weight of tablets depends also on the density of charge of the granulated or loose material in the die as well as on specific weight of the material. Moreover, the weights of the tablets depend on the degree of filling in the die or on the free volume left in the die when the latter is filled with the granulated or loose material introduced therein by the charging or filling device past which each die in the rotary die plate periodically moves. The amount of charge is determined by the position of the die plunger projecting into the die from below and moreover by the manner the filling of the die is carried out or by the speed of charging into the die.
In order to achieve a complete charge of a die during a fast rotation of the die plate, known tablet presses are equipped with a charging or filling wheel and with a dosing wheel, both wheels being driven for rotation in counter-direction relative to each other. Underneath the filling wheel is located a rail-like cam and underneath the dosing wheel is located an elevated rail-like dosing cam. The both cams can be united to form a one-piece rail that is vertically adjustable (German publication No. 22 51 832). The two rails of course can remain as two separate pieces so that the dosing cam is vertically adjustable relative to the filling cam.
In the manufacture of a tablet when the die moves past the filling device, the corresponding lower die plunger in the range of the filling wheel is held at a lower position by means of the filling cam and in the subsequent range below the dosing wheel is raised by running up on the dosing cam. Accordingly as soon as the filling wheel in the first-mentioned range charges the loose tablet material in the die, in the subsequent second range the charged material is lifted in the die by the action of the lower die plunger and a portion thereof is discharged upwardly and wiped off the upper surface of the die plate by the dosing wheel. In this manner it is insured that each die receives a complete filling. It has been found, however, particularly in the case of fast rotary tablet presses, that the bulk weight or bulk density of the charge material varies within a certain range when the rate of rotation of the filling and dosing wheels as well as of the die plate, is changed. At different rates of rotation of the filling wheel there will result different amounts of the charge and thus of the weight of the charge. The weights of respective charges depend, therefore, on the adjustable rate of rotation both of the filling wheel and the dosing wheel (both having usually a uniform rotary speed), and on the vertical position of the lower die plungers set by vertical adjustment both of the filling and dosing cams or exclusively by the vertical adjustment of the dosing cam.
From the German publication 22 51 832 it is known how to compare an actual value of the measured compressing force with a nominal value thereof and how to adjust in response to the ascertained difference the weight of the tablet by adjusting vertically the dosing cam. It is true that by such vertical adjustment of the dosing cam the weight of tablets can be more effectively influenced then by changing the rate of rotation of the filling wheel. Different rates of rotation of the filling wheel, however, still have a considerable influence on the weight inasmuch as at certain speeds of the filling wheel considerable weight variations may occur among consecutively manufactured tablets whereas uniform tablet weights are obtained in a very narrow range of rotary speeds of the filling wheel. It is, therefore, advantageous to make use of those rotary speeds of the filling wheel which produce lowest variations of the charge weight. The optimum rotational speeds, however, are again dependent on the size of the die or on the position of the lower die plunger.
In the aforementioned known method of regulating the weight of tablets by regulating the compressing forces there is first computed from a succession of compressing forces a mean or average value that is compared with predetermined nominal limits and in the case when the measured forces exceed those limits the machine is stopped. This known kind of control, however, does not have proved to be completely satisfactory. For example, if three consecutively measured values 10, 11 and 12 are combined into the mean value 11 and if the nominal limits are set to be 10 and 12 so both the mean value and also all individual components lie within the predetermined limits. If, however, under different circumstances values 1, 11 and 21 are measured, there also results a mean value 11 lying between the set limits in spite of the fact that two of the three measured individual values widely exceed the permissible limits. This known method does not, therefore, take into account larger variations of compressing forces and these variations cannot be recognized from the resulting mean value. On the other hand, if in the known method an undesirable deviation between the actual value and the nominal value of the measured forces is ascertained, an automatic regulation or a manual vertical adjustment of the lower die plunger or of the dosing cam will follow. During this action variations of compressing forces and thus variations of tablet weight resulting from each change of rotary speed of the filling wheel could not or had not been taken into account. Besides, variations of the mean value of compressing forces can be compensated only with a time delay so that the range of variations of individual compressing forces cannot be substantially influenced by employing the above-described prior art method.